It has been shown that provoked diaphragmatic contraction could reverse the pleural pressure (Ppl) gradient (38,41). Furthermore, it has been shown that in the supine dogs subjected to unilateral electrophrenic respiration (40) ventilation distribution was maintained so that normal gas exchange was achieved despite paradoxical regional chest expansion and uneven transpulmonary pressures (Ptp.s). Lobar interdependence was proposed as the mechanism which preserved stable ventilation distribution under these conditions of unusual thoracic configuration and Ptp. gradient (41a). These new findings raise important questions related to regional lung mechanics (Ppl gradient and ventilation distribution) which in the past, have been attributed to gravity effects alone. In the project described here we propose: a) to study in depth the role of diaphragmatic contractions, spontaneous and provoked, in the determination of Ppl gradient; b) to further define the lobar interdependence concept and the potential role of a free pleural cavity in the mechanical regulation of ventilation distribution; c) to evaluate the mechanical limitations to the respiratory system imposed by pleural symphysis that would impede lobar interdependence; and d) to attempt partition of Ppl into two components, one related to lung volume and one to pulmonary distortion, and analyze their interaction in stabilization of ventilation despite large variations in regional Ptp.s. Techniques involved in this research proposal have been tested extensively in our previous work and recently published in detail (39-41). We believe that our investigation into the new areas of diaphragmatic contractions as a gravity independent Ppl determinant and into lobar interdependence as a mechanism for maintenance of stable ventilation distribution, will expand our understanding of regional lung mechanics, with implications for both clinical problems and conceptual analysis of respiratory performance.